Chemical vapor deposition of uranium and plutonium

ABSTRACT

URANIUM (U) OR PLUTONIUM (PU) IS DEPOSITED UPON A SUBSTRATE BY VAPORIZING TRISCYCLOPENTADIENYLURANIUM (IV) CHLORIDE (CP2UCI) OR PLUTONIUM (III) TRICYCLOPENTADIENIDE (PUCP3), HEATING THE SUBSTRATE UPON WHICH THE PU OR U IS TO BE DEPOSITED TO FROM ABOUT 600*C. TO ABOUT 900*C., AND CONTACTING THE VAPORIZED COMPOUND WITH THE HEATED SUBSTRATE TO DEPOSIT A COATING OF U OR PU RESPECTIVELY.

Aug. 27, 1974 v cRlsLER ErAL 3,832,222 CHEMICAL VAPOR DEPOSITION OF URANIUM AND rLuwouIum Filed Dec. 27, 1972 1 "He. 4e 42 44 5s 6 4Q 64 65 A INERTGAS souaca INDUCTION HEATER '24 I a; VACUUM PUMP VACUUM 4 PUMP 82 I=l(5. 2Z 1 United States Patent 3,832,222 CHEMICAL VAPOR DEPOSITION 0F URANIUM AND PLUTONIUM Larry R. Crisler, Arvada, and William G. Eggerman, Boulder, Colo., assiguors to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 27, 1972, Ser. No. 318,807 Int. Cl. C23c 11/02 US. Cl. 117107.2 R 6 Claims ABSTRACT OF THE DISCLOSURE Uranium (U) or plutonium (Pu) is deposited upon a substrate by vaporizing triscyclopentadienyluranium (IV) chloride (Cp UCl) or plutonium (III) tricyclopentadienide (PuCp heating the substrate upon which the Pu or U is to be deposited to from about 600 C. to about 900 C., and contacting the vaporized compound with the heated substrate to deposit a coating of U or Pu respectively.

BACKGROUND OF INVENTION The invention relates to a method of coating a substance or substrate with U or Pu. U or Pu coatings may be useful in such areas as reactor fuel elements and radiation sources to provide thin coatings of radioactive materials. Prior methods for coating a substrate with U or Pu, such as evaporative or sputter deposition using uranium or plutonium metal have sulfered from disadvantages such as the need for high vaporization temperatures or requirements for a line of sight deposition. Other disadvantages of the prior methods include slow deposition rates and limitation on the thickness and purity of the coating which was eifective, as well as poisoning of the decomposition reaction to prevent continuous metal deposition by the formation of a layer of nonvolatile uranium triiodide on the surface of the crude metal as happened when using hot wire deposition in the uranium tetraiodide process.

A method useful in coating a substrate with purified and adherent U or Pu which does away with prior art disadvantages in that only moderate vaporization temperatures are required would be desirable, especially a method which is not dependent upon line of sight deposition parameters or requirements.

SUMMARY OF INVENTION In view of the problems and disadvantages noted hereinabove, it is an object of this invention to provide a new and improved method of depositing U or Pu using vapor deposition processes.

It is a further object of this invention to provide a process which yields relatively high deposition rates of U or Pu.

It is a further object to provide a process for coating a substrate with U or Pu which does not require excessively high vaporization temperatures and which is not dependent upon line of sight deposition parameters.

It is a further object of this invention to provide a process which may be used for producing U and Pu coatings on irregularly shaped substrate configurations.

Various other objects and advantages will appear from the following description of the invention and are particularly pointed out hereinafter in connection with the appended claims. Various changes in design, materials, etc., as described herein may be made by those skilled in the art without departing from the scope and principles of this invention as brought out in the appended claims.

The invention comprises heating a substrate upon which the U or Pu is to be deposited to a temperature of from about 600 C. to about 900 C., vaporizing Cp UCl or 3,832,222 Patented Aug. 21, 1974 PuCp and contacting the vapors of said sublimed compound with said heated substrate to effect deposition of U or Pu respectively.

DESCRIPTION OF DRAWING FIG. 1 is a cross section and block view of an apparatus suitable for effecting this invention.

FIG. 2 is a cross section and block view of an alternate apparatus for this invention.

DETAILED DESCRIPTION A substrate upon which the U or Pu coating is to be deposited is heated to a temperature of from about 600 C. to aobut 900 C. in a vacuum at a pressure less than about 10- torr or in an inert atmosphere taken from the group consisting of argon or helium at a pressure of less than 10- torr. Either Cp UCl or PuCp (Cp being C H may be vaporized or sublimed under the same pressure as above by heating the material to from about 175 C. to about 250 C. for Cp UCl or from about C. to about C. for PuCp Reference to vaporize as used herein is to be understood to include such other terms as sublime. The vapors generated, or sublimed material, may be subsequently maintained at a temperature within the range of from about C. to about 250 C., and preferably at about 250 C. for Cp UCl, and about 140 C. to about 160 C. and preferably about 160 C. for 'PuCp If the substrate upon which the U or Pu is to be deposited is in close proximity to the compound that is being sublimed, measures to maintain the temperature at about 250 C. may not be required. As the vapors come in contact with the heated substrate, a pyrolytic reaction occurs resulting in uranium or plutonium dissociation from the vaporized or sublimed compound and deposition upon the substrate. The amount of deposition as well as the deposition rates may be varied by adjusting the length of time the substrate is exposed to the vapors as well as by adjusting the heat input to the compound being vaporized, effecting increased sublimation rates thereby. FIG. 1 shows apparatus 10 which may be used for effecting this invention and the apparatus will be hereinafter described.

The material 12 to be sublimed or vaporized may be placed in a suitable container or receptable 14, such as a Schlenk tube, through an appropriate material feed or input port 15. After the material is in container 14, valve 17 may be closed so as to prevent removal or escape of vapors through port 15. The system 10 may be then appropriately evacuated, such as by proper controlling of valve 20 to allow passage of any vapors or gas from the various chambers and conduits of apparatus 10 out through conduit 30 and vacuum conduit 22 via vacuum pump 24. Conduit 16 and valve 18 may be used to aid in the evacuation of container 14 as indicated by arrow 19 by appropriate controlling of valve 18 which is suitably attached to a vacuum pump (not shown). The apparatus 10 may be evacuated to the pressure recited hereinabove. After evacuation to the desired level, the system or apparatus 10 may thereafter be backfilled to about atmospheric pressure with a suitable inert gas such as argon, nitrogen or helium which is fed in from inert gas source 26 through conduit 28 and properly controlled valve 20 to allow the gas to enter through conduit 30 into the various chambers and conduits of apparatus 10. Backfilling may be conducted to insure complete removal of other gases from system 10. After the inert gas flush, the system may again be evacuated as stated hereinabove. The material 12 to be vaporized, which may be Cp UCl or PuCp is then heated by an electrical resistance or the like heater 34. The vaporized material may travel or pass through conduit 36 into a transport chamber or container 38 which may be formed by the inner walls of temperature-control jacket 40. Transport chamber 38 may be separated from the deposition chamber 44 wherein deposition occurs by walls or constrictions 42 inwardly projecting from the inner walls of temperature control jacket 40. Transport chamber 38 prevents or minimizes a mass of material from striking the surface upon which deposition is occurring in event the material 12 be heated too fast promoting rapid outgassing during heating. Further, chamber 38 facilitates even coating deposition. The vaporized compound will thus pass from transport chamber 38 through opening 46 in wall 42 communicating with deposition chamber 44. Deposition chamber 44 may have supported therein an appropriate billet 48 or the like which may be inductively heated and thus serve to heat the substrate 50. Such a billet 48 may be particularly useful where the substrate cannot be formed of a material which may be inductively or otherwise conveniently heated.

A suitable fluid such as tetradecane (which has a boiling point of about 252 C.) or other suitable temperature control fluid may be refluxed through appropriate conduits or passageways within the temperature control jacket 40 which generally surrounds transport chamber 38 and deposition chamber 44 so that the vapors within these chambers are generally maintained at a suitable temperature such as from about 175 C. to 250 C. and generally at about 250 C. for Cp UCl, or from about 140 C. to 160 C. and generally at about 160 C. for PuCp The tetradecane or other material used for maintaining or retaining the transport chamber 38 and deposition chamber 44 at the desired temperature is input into temperature control jacket 40 at an appropriate port 52 and is removed from the temperature control jacket 40 at port 54 by conventional fluid circulating and temperature control apparatus (not shown). The tetradecane may be previously heated to a suitable temperature to maintain the required temperature range within the temperature control jacket 40 and within transport chamber 38 and deposition chamber 44.

The billet 48 or other suitable material to be heated and retain heat may be heated through any suitable means, e.g., as by inductive heating using induction heater coils 56 appropriately connected to induction heater 58 through electrical conductors 60 and 62 or through other heating means such as resistance heating and the like so as to maintain the deposition probe 64 and the substrate 50 at a temperature of between about 600 and 900 C.

Heating of the material 12 vaporizes the material which is conveyed through the apparatus and across the deposition probe 64. As the compound passes over the substrate 50, it is pyrolyzed, that is, the compound is thermally decomposed and dissociation occurs by the heat of the substrate so as to effectively deposit U or Pu, as the case may be, as a coating upon the substrate 50. The volatile deposition or pyrolysis byproducts are collected at the cool end 65 of housing 32 since they condense at a temperature less than about 140 C.

In the embodiment of the drawing, the substrate 50 is mounted on deposition probe 64 supported within housing unit 32 which may be coupled and sealed to deposition chamber 44 by a flange 66 extending therefrom as shown. The appropriate coupling of housing unit 32 to temperature control jacket 40 and thereafter to container 14 may form a leak proof system which may be effectively evacuated and backfilled as hereinabove described.

The sublimation temperature of the Cp UCl is about 175 C. at about 10* torr and the sublimation temperature of the PuCp is about 140 C. at about 10- torr.

Apparatus 10 may be made from any suitable heat resistant material such as heat resistant glass, ceramics, glass-ceramics or the like. Substrate 50 may be made of any appropriate material such as quartz, suitable heat resistant glass, copper, or stainless steel. Deposition probe 64 shown positioning the substrate also may be made of any appropriate material such as quartz, while billet 48 may be made from any suitable material which may be inductively heated, such as stainless steel, or other steels, iron, or any other suitable billet material which will serve the function of responding to current passed through the induction coils 56 so as to heat the surrounding area. It has been determined that, in general, rough surfaces of substrates (such as at a root-meansquare (RMS) greated than about 10 A. but less than about 25-30 A.) such as quartz yield excellent adherence of vapor deposited uranium.

Table I iliustrates the composition of a typical coating formed as described above at about 800 C. on a quartz substrate, as determined by spark source mass spectros copy.

TABLE I.-ELEMENTAL ANALYSIS OF COATING BY SPARK IVLASS SPECTROSCOPY Atom, Weight percent percent FIG. 2 is a cross section and block view of a simple alternate apparatus 70 for effecting this invention. It comprises a heat resistant tube 72 or the like used for typical sublimation and pyrolysis of Cp UCl or PuCp material 74 disposed within tube 72. Electrical resistance heater 76 sublimes the material 74. The coating is intended to deposit upon a middle portion 78 of tube 72. Middle portion 78 is heated to from about 600 C. to about 900 C. and preferably to about 800 C. by an appropriate heater 80 such as an electrical resistance heater. The tube is evacuated by means of conduit 82 interconnected with vacuum pump 84.

In a typical process run using apparatus 70, a suitable quantity of Cp UCl material 74 such as between about 5 and 20 milligrams, is placed in tube 72. The tube may then be evacuated to about 5 to 6 10- torr. The middle portion 78 of tube 72 may then be heated by heater 80 to from about 600 to about 900 C. followed by heating of the sample 74 to from about C. to about 250 C., or until a pressure rise to about 5 l0 torr indicates the start of sublimation. The sublimed material may pyrolyze at the high temperature to deposit a uranium coating on the inside surface of the tube 72 at middle portion 78. The carbonaceous byproducts are then sublimed further and condensed at about 50 C. as a brown-black gummy residue in an upper portion 86 of tube 72. An about 10 micron thick layer of uranium on the tube 72 may be effected in about 20 minutes yielding a deposition rate of about 0.5 a/min.

Uranium coatings as well as plutonium coatings may be deposited by chemical vapor deposition of Cp UCl or PuCp respectively. These coatings may range from tWo to three microns thick depending on the rate and time of deposition. Factors which affect the coating characteristics such as uniformity and coating of irregular configurations are such as heating rates and vacuum pressures.

Coating rates to achieve a thickness of from 2 to 3 microns are such as about 0.5 microns per minute. The preferred thickness using this method will vary upon the need for the coating thickness required but generally gOOd coatings have been deposited of about 3 to about 10 microns thickness. Factors that will govern the coating rates are such as pressure, volatilization and deposition temperatures. Coatings made by this method have such properties as being very uniform in coating thickness as determined by test such as examination of cross sections by electron microscopy.

Sublimation of the Cp UCl or PuCp provides uniform even coatings upon the preheated substrate since the sublimed material will envelope or surround the substrate and dissociate thereon. Therefore this invention is not dependent upon line of sight requirements and may be successfully used to coat surfaces not directly exposed in line of sight with the material being sublimed.

What is claimed is:

1. A method for providing on a substrate a coating of at least one of the substances uranium and plutonium, comprising evacuating an atmosphere about said substrate and about a material selected from the group consisting of triscyclopentadienyluranium (IV) chloride and plutonium (III) tricyclopentadienide to a pressure between about and about 10- torr, purging said atmosphere with an inert gas selected from the group consisting of argon, nitrogen, and helium, thereafter vaporizing said material under a pressure of from about 10" to about 10- torr, heating said substrate to a temperature between about 600 C. and about 900 C. and flowing the vapor toward said substrate, and thereafter constricting said flow in advance of said substrate to minimize material striking said substrate, and subsequently contacting said substrate with said vaporized material eifecting deposition of said substance upon said substrate.

2. The method of claim 1 including maintaining said vaporized triscyclopentadienyluranium (IV) chloride ma- 6 terial at a temperature between about 175 C. and about 250 C.

3. The method of claim 1 including maintaining said vaporized triscyclopentadienyluranium (IV) chloride material at a temperature about 250 C.

4. The method of claim 1 including maintaining said pressure at about 1X10" torr.

5. The method of claim 1 includin maintaining said vaporized plutonium (III) tricyclopentadienide material at a temperature between about C. and about C.

6. The method of claim 1 including maintaining said vaporized plutonium (III) tricyclopentadienide material at a temperature about 160 C.

References Cited UNITED STATES PATENTS 2,898,235 8/1959 Bulloff l17106 R X 3,031,338 4/1962 Bourdeau 117-107.2 R X 3,488,367 1/1970 Fischer et a1. 260429.1

LEON D. ROSDOL, Primary Examiner H. A. PITLICK, Assistant Examiner 

